1. Field of the Invention
The present invention relates to a zoom lens suitable for a projection lens which is used for an image projection apparatus such as a liquid crystal projector.
2. Description of the Related Art
In an image projection apparatus which modulates each of lights of three colors such as R, G, and B using a light modulation device such as a liquid crystal panel to synthesize and project the modulated lights, a color synthesis device (a dichroic film, or an optical device having a polarization separating film) for synthesizing the lights of the three color is arranged between the light modulation device and a projection lens. Therefore, the projection lens (zoom lens) preferably has a long back focus and is preferably telecentric at a liquid crystal panel side. Further, because the image projection apparatus is required for obtaining a higher zoom ratio and a higher luminance of a projection image, a projection lens with small (bright) F-number and with high zoom ratio is required.
U.S. Pat. No. 6,785,055, and Japanese Patent Laid-open Nos. 2008-052174 and 2004-085979 disclose a projection lens with a small F-number change in accordance with zooming.
However, the projection lens disclosed in U.S. Pat. No. 6,785,055 and Japanese Patent Laid-open No. 2008-052174 has a small zoom ratio, and recent needs for a high variable magnification are not satisfied.
The projection lens disclosed in Japanese Patent Laid-open No. 2004-085979 is provided with a stop (a variable stop) whose opening diameter is variable to reduce the change of the F-number in accordance with the zooming, but the configuration of the projection lens is complicated by providing the variable stop.
When the modulated three colored lights are projected on a plane to be projected, the three colored projection images formed by the three colored lights are required to precisely overlap with one another. Therefore, a color displacement (chromatic aberration of magnification) generated by the projection lens needs to be appropriately corrected in a visible light range. Further, a distortion by the projection lens needs to be appropriately corrected so that an outline of the projection image is not distorted.